Cyanine dye is an important organic dye, which can be applied to a variety of fields, such as textile, photosensitive science, excimer lasers, fluorescent probes and optical recording media. The first cyanine dye, synthesized by Grevill Williams in 1856, was originally used as a textile dye. Thereafter, in 1875 Vogel discovered the cyanine dye having special photosensitizing properties and thus being suitable to be used as a photosensitive material. Subsequently, the Cyanine dye was first proposed as a recording medium in optical disc by Law et al. (K. Y. Law, P. S. Vincett and G. E. Johnson, Appl. Phys. Lett., 39, 718 (1981)). Because the high molar extinction coefficient of the cyanine dye, the recording layer on an optical disc can be ablated clear pits even by applying a little energy (about 0.5 NJ/bit) and can impart the recording layer a higher signal noise ratio with relative high n value and suitable k value (see U.S. Pat. No. 5,976,658 and U.S. Pat. No. 5,773,193), and thereby produce high quality recordable discs (such as CD-R and DVD-R). The CD-R international standard (Orange Book) was initially established in accordance with the typical properties of the cyanine dye, and most of the CD-R burners were also designed by reference to the property of the cyanine dye as well. Later, phthalocyanine and azo dyes were continuously developed and used in CD-R recording media. Briefly speaking, the organic dyes are used as storage material in the recordable discs. The recording material containing such organic dyes can be coated onto the substrate through spin coating, which can shorten the process and reduce the cost as compared with the vacuum evaporation deposition method. Therefore, the stability of the organic dye and its solubility in organic solvent are very important.
For new generation of information and multimedia, storage media of high capacity, small in size and low production cost is demanded. Therefore, how to increase the storage density of optical storage media is one goal for research and development. CD-Rs possess the advantages of cheap in price, high burning and writing speed, convenient to carry and excellent compatibility with personal computers. As for the optical storage media, one of the most important processes which has been successfully developed is to shorten the reading and writing wavelength of the laser from the optical pickup head. For example, changing the pickup's laser from infrared laser to red light laser can be achieved by increasing the numerical aperture of the lens so that a recording media having high storage density, such as DVD-R disc is accordingly used. The wavelength for recording and playing DVD-R is about 650 nm, and its recording capacity of DVD-R is seven times of that of CD-R disc.
Dyes used for DVD-R should have adequate absorption at a wavelength of 650 nm and good solubility in organic solvents, especially in alcohol solvents. Among them, fluoroalcohols (such as 2,2,3,3-tetrafluoro-1-propanol, TFP) are the most commonly used solvents in the optical disc preparation process. The dye is required to have sufficiently high reflectivity and stability upon forming into a film in order to have a long term data storage stability. Other than the absorption at the desire wavelength, increasing the writing speed of DVD-R from low speed (1×˜8×) to high speed (12×˜16×) also required. The properties of the recording layer become even more important when the recoding disc is recorded at a much high writing speed. This is because if the optical disc is written at high writing speed, the shortened writing time thus requires the media to have much higher sensitivity. In addition, high writing speed accomplished by using high power laser irradiation will results in narrower in the writing margin along with problems especially poor jitter feature, increasing of the laser power and so on. Therefore, much efforts have devoted to develop novel recording material by modifying the structure of the dye such that the thermal decomposition property and ablating speed of DVD-R optical disc can be improved. Generally, the dye used in optical recording media possesses a conjugated system in the molecule as a main characteristic, wherein the skeleton of the chemical structure determines their main absorption range. Semi-empirical molecular orbital calculation methods, such as EHMO, PPP, CNDO and MNDO, can be used in designing new dye. Among them, the PPP method is considered to be the best method for calculating the absorption wavelength of the dye molecules. Since the relationship between the molecular structure of the dye and its other properties (especially thermal performance) is still not well established, selection of dyes suitable for recording material still relies on experimentation.
Among the dye for use in the recording layer of the DVD-R optical disc, cyanine dye has better competitive advantages in the market due to its good coating processability and good solubility in solvents. Prior art such as Taiwan Patent Publication No. 200706602 and No. 200736345 discloses cyanine dimer compounds having improved ablating properties. The novel trimethine dimer compound for use as recording media disclosed in Taiwan Patent Publication No. 200706602 has good characteristics of high recording speed, excellent sensitivity and small thermal interference when forming pits. Taiwan Patent Publication No. 200736345 discloses methine dyes which have a bis-indolenine skeleton and can absorb light in the ultra violet and the infrared regions. The methine dyes have excellent light stability, heat resistance, low jitter and excellent sensitivity. In addition, compared to conventionally known relevant compounds of having solubility in TFP (tetrafluoropropanol) being less than 1.10 wt. %, the solubility of the compound disclosed in Taiwan Patent Publication No. 200736345 is up to 2.68 wt. %.
In recent years, the global economy is booming, but the mineral resource is relatively limited, which results in soaring price of raw material. TFP, the essential solvent used in the optical disc manufacturing process, is thus in a skyrocketing price and makes the production cost for optical disc to rise significantly. To reduce the amount of TFP used in the optical disc manufacturing process becomes an important issue to be resolve. Therefore, there is a great demand for developing optical disc dyes having excellent solubility in this solvent.